Normal neutron activation analysis measures the average concentration of one or more analytes in a single analysis volume. Neutron activation analysis is an extremely powerful method for measuring major, minor, and trace element concentrations in a wide variety of samples. Analyte elements absorb a neutron to form a radionuclide which usually decays by emitting a .beta.-particle and a .gamma.-ray. The .gamma.-ray energies are characteristic of the analyte element and they are normally measured with a germanium detector. Modern germanium crystal .gamma.-ray detectors have excellent energy resolution which provides for simultaneous in situ determination of many elements. This procedure, performed without chemical separations, is called instrumental neutron activation analysis (INAA). Although the INAA takes place on elements located in situ within unaltered samples, information on the three-dimensional locations of the elements is never acquired.
Beta-electrons provide a method for gathering lateral position information for individual radionuclide decompositions in thin samples or particles. Neutron activated nuclides usually decompose by .beta.-decay, effectively producing a nucleus in which a neutron has been converted to a proton. The nucleus emits a neutrino, and usually a .gamma.-ray in addition to the .beta.-electron. The emitted electrons have substantial energies which are largely expended in the production of secondary electrons. Secondary electrons with energies of a few electron volts can be imaged if they pass out of the sample.